The present invention relates to an image information reading system for reading and reproducing image information in facsimile apparatus.
In the conventional image information reading system employing a solid scanning element for scanning one line, two types of systems are known in the art, that is, a system in which the subscanning is performed each time the main scanning of one line has been completed, and a system in which the main scanning and the subscanning are performed simultaneously.
In the former system, it could happen that information which may exist between one main scanning line and the next main scanning line is missed, and thin lines in the main scanning direction of the picture to be transmistted are not reproduced. Therefore, recently ony the latter system is in general use.
In the image information reading system in which the main scanning and the subscanning are performed simultaneously, images cannot be reproduced well if the scanning time for one main scanning line is set too long.
This is because there is a relationship between a charge storage time .tau..sub.D and a dark signal S.sub.D in the solid scanning element as shown in FIG. 1. When the scanning time is set too long, the dark signal S.sub.D is increased and, at the same time, the dark signal S.sub.D is stored in the solid scanning element, together with the image signal, making the S/N ratio inferior. It follows that erroneous information tends to be contained in the image information output from the solid scanning element.
Conventionally, in order to improve the S/N ratio by shortening the charge integration time .tau..sub.S, the scanning time is increased by N times and, as shown in FIG. 2 (a), the image information for one line is obtained by scanning in 1/N of the time .tau. required for reading one line (the time available for the main scanning). In another conventional methods, as shown in FIG. 2 (b), the read scanning is performed N times within the time .tau. available for reading one line, and the image information for one line is obtained from the scanning out of the N scannings.
As a result, the S/N ratio is improved, but the main scanning is performed only for 1/N of the reading time .tau. for one line, while the subscanning is performed continuously during the reading time .tau., so that the resolution in the subscanning direction is reduced, and thin lines in the main scanning direction may be omitted. This makes it difficult to obtain good images in the conventional image information reading system.